1. Field of the Invention
The present invention relates to a resin film laminated metal sheet to be mainly used to drums and caps of food stuffed cans, in particular a resin film laminated metal sheet for can, which has excellent formability and adhesion while can-making and superior taking-out property of stuffed food contents, and to a method for fabricating the same.
2. Description of Related Art
Conventionally, coatings have been carried out on metal sheets such as tin free steel (TFS) or aluminum as blank materials for can to be used to food stuffed cans. The coating technique was involved with many problems of being complicated in a baking procedure, taking much treating time, or exhausting much solvent. Therefore, instead of coating, a lot of methods have been proposed for laminating a thermoplastic resin film to heated metal sheets.
The object of these methods is to improve formability and adhesion of resin film laminated metal sheets, mainly {circle around (1)} by applying a resin film having a polar group such as a polyester resin (for example Japanese Patent Laid Open No. 63-236640) or {circle around (2)} by carrying out a treatment such as corona discharge on a surface of resin film so as to increase a surface free energy xcex3s of resin film (for example, Japanese Patent Laid Open No. 5-200961). In particular, Japanese Patent Laid Open No. 5-200961 discloses that the xcex3s of resin film should be specified in a range of 38 to 54 dyn/cm for securing adhesion after forming polyethylene film laminated metal sheets.
On the other hand, there is a problem that if such a resin film laminated metal sheet is applied to food stuffed cans, when removing fully stuffed food contents from the can, it is difficult to take out them because they are firmly stuck to an inside of the can. This problem weakens consumers"" purchasing desires, and a resolution of the problem is seriously important, however up to now no investigation has been ever performed.
It is accordingly an object of the invention to provide a resin film laminated metal sheet for can, which is excellent in formability and adhesion while can-making and in taking-out property of stuffed food contents, and a method for fabricating the same.
This object can be accomplished by such a resin film laminated metal sheet for can, in which both faces of the metal sheet have resin film laminated layers, and a surface free energy xcex3s of a face of the resin film is 10 dyn/cm or more to less than 30 dyn/cm, the face becoming an inside of can after can-making and being contacted with stuffed food contents.
As the resin film, available is, for example, a polypropylene film or a propylene ethylene based random copolymer film of polypropylene being a main component.
It is more effective to use a resin film of polyester being a main component and contain a wax component of 0.1 to 2.0% in the resin film which will be an inside of can after can-making.
The resin film laminated metal sheet for can having such a resin film may be fabricated by a method comprising a step of laminating a resin film composed of a polypropylene film or a propylene ethylene based random copolymer film of polypropylene being a main component on the surface of the metal sheet which will become an inside of can after can-making, wherein the temperature of the metal sheet is above the melting point of the resin film after passing laminating rolls; otherwise by another method comprising a step of laminating a resin film of polyester being a main component on the surface of the metal sheet, wherein the temperature of the face of the resin film to be adhered to the metal sheet is above the melting point of the resin film between 1 and 20 msec.